1. Field of the Invention
The present general inventive concept relates to a silicon direct bonding (SDB) method, and more particularly, to a silicon direct bonding (SDB) method by which void formation caused by gases is suppressed.
2. Description of the Related Art
In general, a silicon substrate called a ‘wafer’ is used in the manufacture of various semiconductor devices. That is, various semiconductor devices are formed after micromachining like forming a predetermined material layer on such a silicon substrate or etching the surface of the silicon substrate.
In such a process of manufacturing a semiconductor device, two silicon substrates are bonded to each other. In this case, a silicon direct bonding (SDB) method has been generally used to bond two silicon substrates. Generally, the SDB method undergoes the following operations. Firstly, after two silicon substrates are prepared, the substrates are cleaned. Then, a thin film formed of ions and molecules such as OH−, H+, H2O, H2 and O2 is formed on each of bonding surfaces of the two substrates. Subsequently, if the two substrates closely contact, the two substrates are pre-bonded with a force (Van der Waals's force) between the ions and the molecules. Next, if the two substrates in the close-contact state are put into a thermal treatment furnace and heated up to approximately 1000° C., the two substrates are strongly bonded with inter-diffusion between atoms of the two substrates.
However, gases are generated by the ions and the molecules that exist between the two substrates when the two substrates are thermally treated, and these gases may not be completely discharged during the thermal treatment, and therefore may remain between the two substrates. In this case, owing to the gases, a plurality of voids are formed in junctions of the two substrates. The voids cause a bond strength between two silicon substrates to be lowered and an inferiority rate of a semiconductor device formed on the substrates to be increased, such that a yield is lowered. This problem occurs even more often when the size of a substrate is increased and a bonding area is large.